Sale of -400s to Cathay has been rumoured for a while, but would I think be unlikely in the near future since BA are unlikely to make large purchases currently while they consolidate post-FSS, and the more likely sale remains the 777 with a larger sale value, and larger market demand. The -400 would be unlikely to achieve as much cash relatively (accounting for age and depreciation) simply as it is not in demand. So why bother to sell what is effectively the backbone of the LH fleet, when lighter 777 routes could be replaced with 7E7s?
I don't think the ETOPS consideration is much of an issue with the 777 going to Tokyo and Buenos Aires anyway, which are possibly more isolated than any other route - Barbados does have the Azores between it and London and has been operated largely by the 777 for a long time now.
The real case for new aircraft would be if BA were to decide to resume anything more than marginal growth of their ( profitable) long haul business and therefore need a larger fleet.
What Skylion says is possibly quite true - we all know the focus is hardly on SH from London due to the profit margins available.
Plus you must remember that all the business traveller love the -400 for the upper deck, the exclusive First cabin etc. What do BA have left as a marketing tool to make it different from the others with Concorde gone - that can only be a distinguishing characteristic for so long.